Real Estate Properties - Summary of Properties Sold (Details) $ in Thousands		1 Months Ended						3 Months Ended		6 Months Ended
	Jun. 27, 2014 property building	Jun. 30, 2015 USD ($) ft² property building	May. 31, 2015 USD ($) ft² property building	Apr. 30, 2015 USD ($) ft² property building	Mar. 31, 2015 USD ($) ft² property building	Jan. 31, 2015 USD ($) ft² property building	Jan. 31, 2013 property building	Jun. 30, 2015 USD ($) ft² property building	Jun. 30, 2014 USD ($) ft²	Jun. 30, 2015 USD ($) ft² building	Jun. 30, 2014 USD ($) ft²
Real Estate Properties [Line Items]
Number of Properties | property							3
Number of Buildings							18
Square Footage (in sqft) | ft²		29,385,000						29,385,000	42,920,000	29,385,000	42,920,000
Gain (Loss) on Sale | $								$ (2,708)	$ 0	$ 3,160	$ 0
333 Laurel Oak Drive
Real Estate Properties [Line Items]
Number of Buildings										3
1000 Vorhees Drive and 400 Laurel Oak Drive
Real Estate Properties [Line Items]
Number of Buildings										3
Disposed of by Sale
Real Estate Properties [Line Items]
Number of Properties | property								68
Number of Buildings								83
Square Footage (in sqft) | ft²		10,834,604						10,834,604		10,834,604
Gross Sales Price | $								$ 1,087,388
Gain (Loss) on Sale | $								$ 3,160
Disposed of by Sale | 333 Laurel Oak Drive
Real Estate Properties [Line Items]
Number of Buildings			2		1
Disposed of by Sale | 1000 Vorhees Drive and 400 Laurel Oak Drive
Real Estate Properties [Line Items]
Number of Buildings			2		1
Disposed of by Sale | Portfolio of Small Office and Industrial Assets
Real Estate Properties [Line Items]
Number of Properties | property			45
Number of Buildings			53
Square Footage (in sqft) | ft²			5,287,790
Gross Sales Price | $			$ 376,000
Gain (Loss) on Sale | $			$ (7,915)
Disposed of by Sale | AL, LA, NC, SC Office Portfolio
Real Estate Properties [Line Items]
Number of Properties | property		6
Number of Buildings		9
Square Footage (in sqft) | ft²		3,031,917						3,031,917		3,031,917
Gross Sales Price | $		$ 417,450
Gain (Loss) on Sale | $		$ 41,596
Disposed of by Sale | St. Louis Portfolio
Real Estate Properties [Line Items]
Number of Properties | property		2
Number of Buildings		2
Square Footage (in sqft) | ft²		165,098						165,098		165,098
Gross Sales Price | $		$ 14,300
Gain (Loss) on Sale | $		$ (2,338)
Disposed of by Sale | Australia Portfolio
Real Estate Properties [Line Items]
Number of Properties | property								11
Number of Buildings								11
Square Footage (in sqft) | ft²		1,758,525						1,758,525		1,758,525
Gross Sales Price | $								$ 232,955
Gain (Loss) on Sale | $								$ (47,804)
Suburban property | Disposed of by Sale
Real Estate Properties [Line Items]
Number of Properties | property	13
Number of Buildings	41
Suburban property | Disposed of by Sale | 11350 North Meridian Street
Real Estate Properties [Line Items]
Number of Properties | property						1
Number of Buildings						1
Square Footage (in sqft) | ft²						72,264
Gross Sales Price | $						$ 4,200
Gain (Loss) on Sale | $						$ 766
Suburban property | Disposed of by Sale | 333 Laurel Oak Drive
Real Estate Properties [Line Items]
Number of Properties | property					0
Number of Buildings					1
Square Footage (in sqft) | ft²					27,164
Gross Sales Price | $					$ 2,450
Gain (Loss) on Sale | $					$ 251
Suburban property | Disposed of by Sale | 1921 E. Alton Avenue
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					67,846
Gross Sales Price | $					$ 14,533
Gain (Loss) on Sale | $					$ 4,851
Suburban property | Disposed of by Sale | 46 Inverness Center Parkway
Real Estate Properties [Line Items]
Number of Properties | property				0
Number of Buildings				0
Square Footage (in sqft) | ft²				0
Gross Sales Price | $				$ 2,000
Gain (Loss) on Sale | $				$ 1,857
Suburban property | Disposed of by Sale | Sorrento Valley Business Park
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		4
Square Footage (in sqft) | ft²		105,003						105,003		105,003
Gross Sales Price | $		$ 23,500
Gain (Loss) on Sale | $		$ 11,896
Suburban property | Disposed of by Sale | 7450 Campus Drive
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			77,411
Suburban property | Disposed of by Sale | 129 Worthington Ridge Road
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			227,500
Suburban property | Disposed of by Sale | 599 Research Parkway
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			48,249
Suburban property | Disposed of by Sale | 181 Marsh Hill Road
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			162,036
Suburban property | Disposed of by Sale | 101 Barnes Road
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			45,755
Suburban property | Disposed of by Sale | 15 Sterling Drive
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			173,015
Suburban property | Disposed of by Sale | 35 Thorpe Avenue
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			79,862
Suburban property | Disposed of by Sale | 50 Barnes Industrial Road North
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			154,255
Suburban property | Disposed of by Sale | 5-9 Barnes Industrial Road
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			38,006
Suburban property | Disposed of by Sale | 860 North Main Street
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			31,165
Suburban property | Disposed of by Sale | One Barnes Industrial Road South
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			30,170
Suburban property | Disposed of by Sale | Village Lane
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			2
Square Footage (in sqft) | ft²			58,185
Suburban property | Disposed of by Sale | 100 Northfield Drive
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			116,986
Suburban property | Disposed of by Sale | 905 Meridian Lake Drive
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			74,652
Suburban property | Disposed of by Sale | 1717 Deerfield Road
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			141,186
Suburban property | Disposed of by Sale | 1955 West Field Court
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			59,130
Suburban property | Disposed of by Sale | 5015 S. Water Circle
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			113,524
Suburban property | Disposed of by Sale | Adams Place
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			2
Square Footage (in sqft) | ft²			230,259
Suburban property | Disposed of by Sale | Cabot Business Park
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			2
Square Footage (in sqft) | ft²			252,755
Suburban property | Disposed of by Sale | 2300 Crown Colony Drive
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			45,974
Suburban property | Disposed of by Sale | Myles Standish Industrial Park
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			2
Square Footage (in sqft) | ft²			74,800
Suburban property | Disposed of by Sale | 340 Thompson Road
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			25,000
Suburban property | Disposed of by Sale | 6710 Oxon Hill
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			118,336
Suburban property | Disposed of by Sale | 8800 Queen Avenue South
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			280,822
Suburban property | Disposed of by Sale | 9800 Sherlard Parkway
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			46,765
Suburban property | Disposed of by Sale | Rosedale Corporate Plaza
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			3
Square Footage (in sqft) | ft²			149,116
Suburban property | Disposed of by Sale | 1000 Shelard Parkway
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			62,499
Suburban property | Disposed of by Sale | 1900 Meyer Drury Drive
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			65,225
Suburban property | Disposed of by Sale | 131-165 West Ninth Street
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			75,517
Suburban property | Disposed of by Sale | 7-9 Vreeland Road
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			155,891
Suburban property | Disposed of by Sale | 5 Paragon Drive
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			119,089
Suburban property | Disposed of by Sale | 1000 Vorhees Drive and 400 Laurel Oak Drive
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			2
Square Footage (in sqft) | ft²			125,415
Suburban property | Disposed of by Sale | 1601 Veterans Highway
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			63,608
Suburban property | Disposed of by Sale | Two Corporate Center Drive
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			291,230
Suburban property | Disposed of by Sale | 11311 Cornell Center Drive
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			93,413
Suburban property | Disposed of by Sale | 5300 Kings Island Drive
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			159,421
Suburban property | Disposed of by Sale | 3 Crown Point Court
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			73,987
Suburban property | Disposed of by Sale | 515 Pennsylvania Avenue
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			82,000
Suburban property | Disposed of by Sale | 443 Gulph Road
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			21,000
Suburban property | Disposed of by Sale | 4350 Northern Pike
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			503,885
Suburban property | Disposed of by Sale | Thunderbolt Place
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			2
Square Footage (in sqft) | ft²			100,505
Suburban property | Disposed of by Sale | 6160 Kempsville Circle
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			129,565
Suburban property | Disposed of by Sale | 448 Viking Drive
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			75,374
Suburban property | Disposed of by Sale | Inverness Center
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		4
Square Footage (in sqft) | ft²		475,951						475,951		475,951
Suburban property | Disposed of by Sale | 12655 Olive Boulevard
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		98,588						98,588		98,588
Suburban property | Disposed of by Sale | 1285 Fern Ridge Parkway
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		66,510						66,510		66,510
Suburban property | Disposed of by Sale | 310-314 Invermay Road
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				47,480
Suburban property | Disposed of by Sale | 253-293 George Town Road
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				143,914
Suburban property | Disposed of by Sale | 7 Modal Crescent
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		164,160						164,160		164,160
Suburban property | Disposed of by Sale | 71-93 Whiteside Road
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		303,488						303,488		303,488
Suburban property | Disposed of by Sale | 9-13 Titanium Court
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		69,664						69,664		69,664
Suburban property | Disposed of by Sale | 16 Rodborough Road
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		90,525						90,525		90,525
Suburban property | Disposed of by Sale | 22 Rodborough Road
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		43,427						43,427		43,427
Suburban property | Disposed of by Sale | 127-161 Cherry Lane
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		278,570						278,570		278,570
Suburban property | Disposed of by Sale | 44-46 Mandarin Street
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		226,718						226,718		226,718
Suburban property | Disposed of by Sale | 19 Leadership Way
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		76,714						76,714		76,714
CBD | Disposed of by Sale
Real Estate Properties [Line Items]
Number of Properties | property	1
Number of Buildings	2
CBD | Disposed of by Sale | 100 South Charles Street
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			159,616
CBD | Disposed of by Sale | 525 Park Street
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			75,636
CBD | Disposed of by Sale | 2501 20th Place South
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		125,722						125,722		125,722
CBD | Disposed of by Sale | 420 20th Street North
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		514,893						514,893		514,893
CBD | Disposed of by Sale | 701 Poydras Street
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		1,256,971						1,256,971		1,256,971
CBD | Disposed of by Sale | 300 North Greene Street
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		324,305						324,305		324,305
CBD | Disposed of by Sale | 1320 Main Street
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		334,075						334,075		334,075
CBD | Disposed of by Sale | 310-320 Pitt Street
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		313,865						313,865		313,865
CBD | Foreclosure | 225 Water Street
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			318,997
Gross Sales Price | $			$ 0
Gain (Loss) on Sale | $			$ 0